1. Field of the Invention
The present invention relates to an electric power steering apparatus. More particularly, the present invention relates to an electric power steering apparatus configured to be capable of being assembled in a state in which the tension of a belt is correctly tuned without using a separate process of measuring the tension of the belt, to substantially reduce the number of steps at the time of assembling a motor housing and a rack housing and hence the manufacturing costs, and to reduce vibration and noise to provide a pleasant steering feeling to a driver.
2. Description of the Prior Art
A steering apparatus means an apparatus that allows a driver to change a driving direction of the vehicle by his/her will. The steering apparatus helps the driver to optionally change the center of rotation, about which the vehicle's front wheels are turned, so as to make the vehicle move in a direction desired by the driver.
In order to steer the wheels when the vehicle is in the stopped state, it is necessary to turn the handle with a force to overcome the frictional force between the wheels and a road surface. Specifically, the driver's steering force should be further increased if the vehicle is heavy or if the wheels are wide, since the frictional force between the wheels and the road surface is increased.
In order to reduce the driver's force, a power steering system is additionally provided in such a steering apparatus. A hydraulic power steering apparatuses using hydraulic pressure of a hydraulic pump has been used as a power steering apparatus for a vehicle. However, an electric power steering apparatus using an electric motor has been gradually popularized since the 1990s.
An existing hydraulic power steering apparatus is configured such that a hydraulic pump, which is a power source for assisting power, is always driven regardless whether the steering wheel is rotated or not, thereby always consuming energy. To the contrary, an electric power steering apparatus is configured such that when the steering wheel is rotated to produce a torque, a motor driven by electric energy provides a steering-assisting power. Thus, with such an electric power steering apparatus, it is possible to enhance the energy efficiency as compared to the hydraulic power steering apparatus.
FIG. 1 is a schematic view of a conventional electric power steering apparatus for a vehicle, and FIG. 2 is a side elevational view showing a motor pulley housing and a rack housing in accordance with a prior art.
As illustrated in FIG. 1, an electric power steering apparatus for a vehicle typically includes a steering system 100 that extends from a steering wheel 101 to opposite front wheels 108, and an auxiliary power mechanism 120 that provide auxiliary steering power to the steering system 100.
The steering system 100 includes a steering shaft 102, one end of which is connected to the steering wheel 101 to be rotated together with the steering wheel 101, and the other end of which is connected to a pinion 104 via a pair of universal joints 103. In addition, the pinion shaft 104 is connected to a rack bar 109 through a rack-and-pinion mechanism 105, and each of the opposite ends of the rack bar 109 is connected to one of the vehicle's wheels 108 through a tie rod 106 and a knuckle arm 107.
The rack-and-pinion mechanism 105 is formed by a pinion gear 111 formed on the pinion shaft 104 and a rack gear 112 formed at a side of the periphery of the rack bar 109, in which the pinion gear 111 and the rack gear 112 are engaged with each other. Accordingly, when a driver operates the steering wheel 108, a torque is produced in the steering system 100, and the wheels 109 are steered by the torque transmitted through the rack-and-pinion mechanism 105 and the tie rods 106.
The auxiliary power mechanism 120 includes: a torque sensor 121 that senses the torque applied to the steering wheel 101 by the driver, and outputs an electric signal proportional to the sensed torque; an electric control unit (ECU) 123 that produces a control signal based on the electric signal transmitted from the torque sensor 121; a motor 130 that produces an auxiliary power based on the control signal transmitted from the ECU 123; and a belt-type transmission 140 that transmits the auxiliary power produced by the motor 130 to the rack bar 109 through a belt.
Therefore, the electric power steering apparatus is configured such that the torque produced by rotating the steering wheel 101 is transmitted to the rack bar 109 through the rack-and-pinion mechanism 105, and the auxiliary power produced by the motor 130 based on the produced torque is transmitted to the rack bar 109 via a ball screw unit 150 by the belt-type transmission 140. That is, the torque produced by the steering system 100 and the auxiliary power produced by the motor 130 are summed to make the rack bar 109 move axially.
In addition, as illustrated in FIG. 2, the motor 130 is fastened by joining the motor housing 235 enclosing the motor 130 and a rack housing 207 by bolts 230. At the time of steering, the motor spindle 203 is rotated which in turn drives belt 209 to rotate a ball nut 205. Upon being rotated by the belt 209, the ball nut 205 makes the rack bar 109 reciprocate.
The conventional electric power steering apparatus as described above has a problem in that since it is necessary to measure the tension of the belt when assembling the motor housing and the rack housing, and to tune the tension of the belt, the assembling process is delayed and the manufacturing costs are increased.
In addition, if the electric power steering apparatus were assembled without correctly measuring the belt tension, the electric power steering apparatus may be overloaded, noises may occur between the belt and a driving pulley or a driven pulley, and the belt may be damaged, due to the high or low tension of the belt.